1. Field of the Invention
The present invention relates to a hydraulic booster brake system which energizes a leg-power of a brake pedal by utilizing a hydraulic pressure of a hydraulic pump for a power steering.
2. Description of the Related Art
In order to energize a leg-power of a brake pedal in a medium-size or small-size truck, there may be adopted a hydraulic booster brake system which actuates a brake booster by utilizing a hydraulic pressure generated by a hydraulic pump for a power steering provided in the truck.
In an example of the hydraulic booster brake system, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-167090 published by Japan Patent Office, the brake booster is connected in series to a hydraulic circuit for the power steering. Usually, in this brake system, the brake booster is arranged on the upstream side of a power steering booster in order to give priority to actuation of the brake booster with the great importance. That is, a brake function is first assured by priority when a hydraulic fluid discharged from the hydraulic pump is supplied to the brake booster, and the hydraulic fluid which has passed through the brake booster then flows into the power steering booster, thereby demonstrating a power steering function.
Since the brake booster is arranged on the upstream side of the power steering booster as described above, an interference between the brake function and the power steering function, which is a so-called a system interference may occur on rare occasions depending on operating conditions. Specifically, if the brake booster is actuated by operating the brake pedal when the steering operation is assisted by the power steering booster, the hydraulic fluid required for assisting the steering operation is used for the brake booster, and hence a quantity of flow to the power steering booster becomes temporarily insufficient. As a result, a power required for the steering operation is temporarily increased, and a so-called a steering shock occurs.
Thus, in the prior art, provision of, e.g., suppressing means for suppressing a quantity of flow of the hydraulic fluid flowing into a servo chamber of the brake booster avoids an excessive reduction in a quantity of fluid of the hydraulic fluid flowing into the power steering booster. However, such suppressing means can be a factor of reducing the responsibility of the actuation of the brake booster, and hence it cannot be said that this means is the best countermeasure for avoiding the system interference.
In the brake booster, in order to assure the stable brake performance, a pressure of the hydraulic fluid is set so as not to exceed a predetermined maximum working pressure. The maximum working pressure of the brake booster is set lower than the maximum working pressure of the power steering booster in order to achieve both the brake function and the power steering function. The maximum working pressure of the power steering booster is determined by a relief pressure of the hydraulic pump. If the maximum working pressure of the brake booster is equal to the maximum working pressure of the power steering, the hydraulic fluid flows from a relief valve of the hydraulic pump to a drain side with a pressure lower than the maximum working pressure of the brake booster.
In the prior art, in order to set the maximum working pressure of the brake booster, a choking amount of an inner spool which performs a servo function of the brake booster is adjusted. Further, the maximum working pressure of the brake booster is set lower than the maximum working pressure of the power steering booster.
The hydraulic fluid used in the hydraulic circuit has the viscosity. The hydraulic fluid tends to hardly flow if the viscosity is high. Since the viscosity of the hydraulic fluid varies depending on a temperature, the maximum working pressure of the brake booster varies under the influence of the viscosity of the hydraulic fluid which changes depending on a temperature.
In order to assure a quantity of flow required for the power steering booster by changing a choking amount of the inner spool with respect to irregularities of the viscosity of the hydraulic fluid, a choking amount of the inner spool must be adjusted in such a manner that the maximum working pressure of the brake booster becomes lower than the maximum working pressure of the power steering booster in all temperature ranges in which the brake system is used. On the other hand, there is a demand to set the maximum working pressure of the brake booster close to the upper limit as much as possible in order to improve the brake performance.
However, the maximum working pressure for the brake booster must be set to a value which is far lower than the maximum working pressure of the power steering booster so as not to exceed the maximum working pressure of the power steering booster in all temperature ranges of the hydraulic fluid. Furthermore, since there is a demand to cause a stipulated quantity of the hydraulic fluid to flow into the power steering booster and the brake booster, it is difficult to increase the maximum working pressure of the brake booster.